1. Field Of The Invention
The present invention generally relates to the fabrication of microstructures, such as integrated circuits, and, more particularly, to the operation of ion implantation tools required for producing well-defined doped regions in specified material regions, such as semiconductive regions.
2. Description of the Related Art
The fabrication of complex microstructures, such as sophisticated integrated circuits, requires that a large number of individual process steps be performed to finally obtain the required functionality of the microstructure. Especially in the formation of integrated circuits, the conductivity of specific areas has to be adapted to design requirements. For instance, the conductivity of a semiconductor region may be increased in a well-defined manner by introducing specific impurities, which are also referred to as dopants, and placing some, or preferably all, of these impurities at lattice sites of the semiconductor crystal. In this way, so-called PN junctions may be formed that are essential for obtaining a transistor function, since transistors represent the active elements, i.e., elements providing current or voltage amplification, which are required for manufacturing electronic circuits. In modern integrated circuits, millions of transistor elements, such as field effect transistors, are typically provided on a single die, wherein, in turn, a plurality of dies are provided on a single substrate. As the critical dimensions of certain circuit elements, such as field effect transistors, have now reached 0.1 μm and even less, it is of great importance to correspondingly “fine-tune” the profile of doped regions in the lateral direction, with respect to a substrate, as well as in the depth direction. Commonly, ion implantation is the preferred method for introducing dopants into specified device regions due to the ability to precisely control the number of implanted dopant atoms into substrates with a repeatability and uniformity of better than ±1%. Moreover, impurities that are introduced by ion implantation have a significantly lower lateral distribution when compared to conventional dopant diffusion processes. Since ion implantation is typically a room temperature process, the lateral profiling of a doped region may in many cases conveniently be achieved by providing a correspondingly patterned photoresist mask layer. These characteristics may render ion implantation currently, and in the near future, the preferred technique to produce doped regions in a semiconductor device.
Implantation of dopants is accomplished by various ion implantation tools. Such tools are extremely complex machines that require continuous monitoring of the machine characteristics so as to achieve high efficiency and machine utilization.
With reference to FIG. 1, a schematic overview is given for a typical ion implantation tool and the operation thereof. In FIG. 1, an ion implantation tool 100 comprises an ion source 101 having an input 102 that is connected to respective precursor sources (not shown) from which an appropriate ion species may be created in the ion source 101. The ion source 101 may be configured to establish a plasma atmosphere and to pre-accelerate charged particles into a beam pipe schematically depicted as 103. Downstream of the ion source 101, an accelerator tube 104 is arranged that is dimensioned to accelerate ions with a specified voltage, which may typically range from zero to approximately 200 keV for a typical medium current implanter and may range to several hundred keVs or even to 1 MeV or more in high energy implanters. Next, a beam shaping element 105 such as a quadrupole magnet may be arranged, followed by a deflector magnet 106. Downstream of the deflector magnet 106 is disposed an analyzing aperture, for instance in the form of a slit 107, the dimensions of which substantially determine an energy spread of the ion beam. Thereafter, a further beam shaping element, such as a quadrupole magnetic 108, may be provided downstream of the analyzing slit 107.
A substrate holder 109 is located at the vicinity of the end of the beam line 103, wherein, typically, the substrate holder 109 may be provided in the form of a plate enabling the receipt of one or more substrates 110, wherein the plate 109 is connected to a drive assembly (not shown) that allows moving of the substrate holder 109 in the transverse direction (as indicated by the arrows depicted in FIG. 1) and also allows control of the tilt angle, at least in two planes, at which the ion beam hits the substrate 110. For convenience, corresponding means for controlling and adjusting the tilt angle are not shown. Moreover, a first ion beam detector 111 may be provided, for instance, embodied by a plurality of Faraday cups that are connected with respective current measurement devices. Furthermore, a second ion beam detector 112 may be provided as a so-called traveling Faraday cup that is laterally movable to determine the shape of an ion beam and/or to shade respective Faraday cups during the measurement of specific beam characteristics, such as the angle of incidence. A corresponding arrangement of ion beam detectors 111 and 112 is, for example, realized in an ion implanter VIIsta80®, available from Varian Inc. It should be noted, however, that a plurality of other ion beam detector arrangements are available in other presently available ion implantation tools. A Faraday cup is typically constructed as a conductive container, the interior of which maintains devoid of an electric field when the cup is hit by charged particles. Ibis characteristic enables the detection of an ion beam substantially without influencing the ion beam when moving in the interior of the cup. Typically, a Faraday cup for implantation tools is made of graphite.
During the operation of the ion implantation tool 100, an appropriate precursor gas is supplied by the inlet 102 to the ion source 101 and ions of atoms included in the precursor gas may be accelerated into the beam line 103. Typically, a plurality of different ions having different charge states may be supplied by the ion source 101 and may thus be introduced into the acceleration tube 104. Typically, a preselection of the type of ions as well as of the respective charge states may be performed within the ion source 101 by a corresponding deflector magnet (not shown). Thereafter, the ions pass the accelerator tube 104 and gain speed in accordance with the applied acceleration voltage, the charge states of the respective ion and its corresponding mass. With the quadrupole magnet 105, the ion beam may be focused in one dimension and may be correspondingly defocused in the perpendicular dimension and the correspondingly shaped beam is directed to the deflector magnet 106. The current generating the magnetic field of the deflector magnet 106 is controlled so as to deflect the trajectory of desired ion species having a desired charge state to the opening of the analyzing slit 107. Ions of differing mass and/or charge state will typically hit the analyzer 107 without passing through the slit. Thus, the ions in the beam passing the analyzer 107 have a well-defined mass and an energy distribution defined by the slit size. It should be noted that in some ion implantation tools the deflecting magnet 106 and the analyzer 107 are configured such that the ion beam passing through the analyzer 107 may be scanned in a transverse direction so as to cover the whole area of a substrate or at least a significant portion thereof, since the dimension of the beam shape, i.e., the size of the beam spot, is usually, depending on the energy of the ion beam, significantly less than the area of a substrate to be processed. Next, the beam passing through the analyzer 107 may be further shaped by the quadrupole magnet 108 so that, in combination with the quadruple magnet 105, a desired beam shape may be obtained. The characteristics of the ion beam, i.e., the beam shape, the angle of incidence onto the substrate holder 109 and the internal parallelism, i.e., the beam divergence, and the like, may be measured prior to actually exposing the substrate 110 to the ion beam. To this end, the substrate holder 109 may be removed from the ion beam and the first and/or the second beam detector 111 and 112 may be operated so as to obtain the required measurement results. For instance, the traveling Faraday cup 112 may be positioned at different transverse locations and the corresponding dosage received at each transverse position may be determined so as to estimate and adjust the beam uniformity. Moreover, the Faraday cup 112 may be positioned so as to subsequently shade corresponding Faraday cups of the first ion detector 111, the measurement readings of which may then be used to estimate the main beam incidence angle and the beam divergence. Since both an incorrect angle of incidence and an insufficiently parallel ion beam, i.e., a non-vanishing beam divergence, may compromise a corresponding lateral dopant profile on the substrate 110, it is extremely important to precisely monitor and control the tilt angle and the beam divergence.
It turns out, however, that any change of an implantation parameter, for example the change of any bias voltages of apertures, minor changes of the settings of the beam shaping elements 105 and 108 and the like, require a thorough check of the beam profile and/or of the parallelism and tilt angle, which may necessitate a scan with the traveling Faraday cup, rendering the readjustment procedure extremely time consuming so that production yield and tool utilization is reduced.
In view of the above-identified problems, there exists a need for an improved technique that allows improvement of efficiency and/or accuracy of ion beam monitoring in an implantation tool.